Oxide etching method

ABSTRACT

There is provided an etching method of an amorphous oxide layer containing In and at least one of Ga and Zn, which includes etching the amorphous oxide layer using an etchant containing any one of acetic acid, citric acid, hydrochloric acid, and perchloric acid.

This application claims the benefit of Japanese Patent Application No.2006-209859, filed Aug. 1, 2006, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present invention relates to an etching method of an oxide filmcontaining In, Ga, and Zn with a high selectivity with respect to ITO(Indium Tin Oxide). More specifically, the present invention relates toa highly selective etching method of an oxide containing In and Zn andan oxide containing In and Ga which is employed in producing a fineelectronic component such as a semiconductor device, a semiconductorintegrated circuit, and an electrode.

BACKGROUND ART

In recent years, under circumstances where reductions in size, weight,and power consumption of electronic equipments have been advanced,transparent oxide semiconductors and transparent conductive oxides havebeen drawing attention in a field of display. In particular, since afilm can be formed on a resin film at low temperatures, futureapplication to a lightweight portable electronic equipment and the likeis expected.

As disclosed in Japanese Patent Application Laid-open No. 2004-103957,oxide films containing In, Ga, and Zn are optically transparent, and arealso widely studied for application to transparent electrodes,transistors, and the like by reducing the resistance. Further, H. M. Kimet al., Jpn. J. Appl. Phys. Vol. 42, 2003, p. 223-227 and R. E. Presleyet al., Vol. 50, Issue 3, March 2006, p. 500-503 disclose that reportshave been made on researches on oxide films containing In and Zn oroxide films containing In and Ga.

In addition, researches and developments on oxide films containing In,Ga, and Zn and growing conditions of the films have also been made. Inaddition, production of a semiconductor device using an oxide filmcontaining In, Ga, and Zn on a plastic film is reported by ChienliuChang, et al. in Extended Abstracts of the 53th Spring Meeting of TheJapan Society of Applied Physics and Related Societies, 22a-P1-45/II, p.653, (2006).

Heretofore, patterning of produced oxide films containing In, Ga, andZn, oxide films containing Ga and Zn, and oxide films containing In andZn have been performed by the lift-off process. The lift-off process isdisclosed in K. Nomura et al., Nature, Vol. 432, 25 Nov. 2004, p.488-492, E. M. C. Fortunato et al., Advanced Materials, 2005, 17, No. 5,p. 590-594, P. Barquinha et al., Journal of Non-Crystalline Solid Vol.352, Issues 9-20, 2006, pp. 1749-1752, etc.

For concise description, an oxide film containing In, Ga, and Zn(In—Ga—Zn oxide) is hereinafter referred to as IGZO. Similarly, an oxidefilm containing In and Zn (In—Zn oxide) is hereinafter referred to asIZO. Similarly, an oxide film containing In and Ga (In—Ga oxide) ishereinafter referred to as IGO. When semiconductor devices are producedusing these transparent oxide semiconductors as an active layer,indium-tin oxide (ITO) films are generally used as a transparentconductive electrode.

In Japanese Patent Application Laid-open Nos. S58-120780 and S60-217636,and Japanese Patent Publication No. H04-5756, at least one kind ofsulfuric acid, hydrochloric acid, nitric acid, and ferric chloride isused as a wet etching liquid (also referred to as “etchant”) of ITO. InJapanese Patent Application Laid-open No. 2005-258115, oxalic acid, amixed acid of phosphoric acid, acetic acid, and nitric acid, an aqueouscerium ammonium nitrate solution, or the like is used as an etchingliquid (also referred to as “etchant”) of IZO.

DISCLOSURE OF THE INVENTION

However, the lift-off process as described above has a problem that aphotoresist is melted and deformed at a high temperature. The lift-offprocess has another problem that when a photoresist is removed, an edgeof a pattern of a deposited film is curved up, and thereafter anelectric wire crossing over the pattern edge is liable to be broken,which lowers the production yield. In particular, there is a pollutionproblem that a photoresist is contaminated into a formed film due to alift-off step.

In order to practically use the oxide film containing In, Ga, and Zn forthe above-mentioned various uses, the oxide film containing In, Ga, andZn needs to be processed. Therefore, in order to achieve a desired shapewith high reproducibility, various etching methods have been studied asa patterning method other than the lift-off process. The etching methodis roughly classified into two methods. One is wet etching in which asample is immersed in a chemical agent, and the other one is dry etchingutilizing a gaseous etching medium. Theoretically, the dry etchingmethod has an advantage that chemically active ions generated in aplasma bombard vertically on the surface of the substrate, so that theetched shape is faithfully transferred from the mask pattern. However,maintenance costs for a dry etching apparatus body, a circumferentialgas supply system, exhaust gas processing of a chlorine-based gas, powerconsumption and the like are remarkably large, and there is also aproblem of short life of the apparatus due to particle contamination orthe like. Further, there is a case where the control of highly selectiveetching is difficult due to ion impact. In addition, when consideringfuture production of large area displays, there is a fear that highcapacities and large costs are necessary for a chamber, a vacuum system,and the like of the dry etching apparatus. In contrast, in the wetetching, the etching proceeds isotropically, so that there arises aproblem of generating a phenomenon such as an undercut beneath a mask.However, the wet etching has an advantage that the apparatus cost issignificantly reduced compared with the dry etching. Moreover, the wetetching is considered to be suitable for mass production because of itsease of control and management of concentration and temperatureconditions of an etching liquid (also referred to as “etchant”).

When etching transparent oxides such as the above-mentioned IGZO, IZO,and IGO, the selectivity of an etching liquid to an ITO electrode isvery important.

On the other hand, the composition of the oxide and the film depositionconditions can be adjusted to obtain a large change in the electricalconductivity. Therefore, there can be adopted a stack structure in whicha semiconductor active layer, a high-conductivity electrode layer, andwiring are formed by using only the transparent oxides such as IGZO,IZO, and IGO without using ITO. Therefore, in the case of producing adevice using only the transparent oxides such as IGZO, IZO, and IGOwithout using ITO, the etch selectivity of these oxides becomes a majorsubject. If the etch selectivity is insufficient, there are cases wherea material not to be etched may be etched, which may increase variationin the performance of electronic devices, thus resulting in reduction ofthe yield.

Hitherto, although there have been indications of IGZO and IGO films,there has been no explicit disclosure of etching methods therefor. Theaforementioned documents disclose the etching methods for thetransparent indium oxide film containing IGZO, IZO, and IGO, but have nodescription of the selectivity to an ITO conductive film, for example.As described above, the conventional etching liquids have no selectivitywith regard to differences in oxides. Therefore, when producing a devicein which different kinds of oxides containing indium coexist, there isposed a problem that it is difficult to precisely control the etching.

The present invention has been accomplished to solve the above-mentionedproblems. That is, the present invention provides a method of etching astructure in which IZO and ITO, IGZO and ITO, IGO and ITO, IZO and IGZO,IZO and IGO, or IGZO and IGO coexist. More specifically, the presentinvention provides an etching method of performing selective etching byusing an etching liquid having selectivity for different substances.

The present invention also provides an etching method by which preciseand easy etching can be performed in a thin film semiconductor devicecontaining two or more kinds of oxide films selected from IGZO, IZO,IGO, and ITO.

The present inventor has conducted extensive study on the depositionconditions and etching conditions of films containing IGZO, IZO, or IGO,and thin film semiconductor devices using such films. As a result, thepresent inventor has found that a layer structure including two or morekinds of indium oxide films selected from IGZO, IZO, IGO, and ITO can beselectively etched by using a etching liquid containing any one ofacetic acid, organic acid, hydrochloric acid, or perchloric acid.

The present invention has been accomplished based on the above-mentionedfinding and provides an etching method of an amorphous oxide layercontaining In and at least one of Ga and Zn, which includes etching theamorphous oxide layer using an etching liquid containing any one ofacetic acid, citric acid, hydrochloric acid, and perchloric acid.

Further, the present invention provides an etching method of anamorphous oxide layer containing In and at least one of Ga and Zn in astructure including an ITO layer and the amorphous oxide layer, whichincludes selectively etching the amorphous oxide layer using an etchingliquid containing any one of acetic acid, citric acid, hydrochloricacid, and perchloric acid.

Moreover, the present invention provides an etching method of anamorphous oxide layer containing In and Zn, or In, Ga, and Zn in astructure including an oxide layer containing In and Ga and theamorphous oxide layer, which includes selectively etching the amorphousoxide layer using an etching liquid containing any one of acetic acid,citric acid, hydrochloric acid, and perchloric acid.

In addition, the present invention provides an etching method of anamorphous oxide layer containing In and Zn in a structure including anoxide layer containing In, Ga, and Zn and the amorphous oxide layer,which includes selectively etching the amorphous oxide layer using anetching liquid containing any one of acetic acid, citric acid,hydrochloric acid, and perchloric acid.

According to the present invention described above, precise and highlyselective wet etching can be performed in a thin film semiconductordevice including two or more kinds of oxide films selected from IGZO,IZO, IGO, and ITO. Further, the thin film semiconductor device producedby the etching method of the present invention and by using the etchingliquid having remarkable selectivity of the present invention isadvantageous in that the device performance is not variable and isstabile and uniform, and that the device productions steps are simpleand easy to carry out.

For example, in a production process of a transistor device having atransparent semiconductor oxide film containing IGZO provided asubstrate is used for a channel layer and ITO is used for a drainelectrode and a gate electrode thereof, highly selective etching can beperformed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation illustrating acetic acidconcentration dependence of an etch rate of an indium oxide film, forexplaining an etching liquid containing acetic acid according to Example1 of the method of the present invention;

FIG. 2 is a graphical representation illustrating acetic acidconcentration dependence of an etch selectivity of an indium oxide filmto ITO, for explaining an etching liquid containing acetic acidaccording to Example 1 of the method of the present invention;

FIG. 3 is a graphical representation illustrating room temperaturecitric acid concentration dependence of an etch rate of an indium oxidefilm, for explaining an etching liquid containing citric acid accordingto Example 1 of the method of the present invention;

FIG. 4 is a graphical representation illustrating room temperaturecitric acid concentration dependence of an etch selectivity of an indiumoxide film to ITO, for explaining an etching liquid containing citricacid according to Example 1 of the method of the present invention;

FIG. 5 is a graphical representation illustrating citric acidconcentration dependence at 50° C. of an etch rate of an indium oxidefilm, for explaining an etching liquid containing citric acid accordingto Example 1 of the method of the present invention;

FIG. 6 is a graphical representation illustrating citric acidconcentration dependence at 50° C. of an etch selectivity of an indiumoxide film to ITO, for explaining an etching liquid containing citricacid according to Example 1 of the method of the present invention;

FIG. 7 is a graphical representation illustrating hydrochloric acidconcentration dependence of an etch rate of an indium oxide film, forexplaining an etching liquid containing hydrochloric acid according toExample 1 of the method of the present invention;

FIG. 8 is a graphical representation illustrating hydrochloric acidconcentration dependence of an etch selectivity of an indium oxide filmto ITO, for explaining an etching liquid containing hydrochloric acidaccording to Example 1 of the method of the present invention;

FIG. 9 is a graphical representation illustrating perchloric acidconcentration dependence of an etch rate of an indium oxide film, forexplaining an etching liquid containing perchloric acid according toExample 1 of the method of the present invention;

FIG. 10 is a graphical representation illustrating perchloric acidconcentration dependence of an etch selectivity of an indium oxide filmto ITO, for explaining an etching liquid containing perchloric acidaccording to Example 1 of the method of the present invention;

FIG. 11A is a schematic cross-sectional view of a layer structure beforeetching, for explaining an etching method according to Example 2 of themethod of the present invention, and FIG. 11B is a schematiccross-sectional view of the layer structure after the etching, forexplaining the etching method according to Example 2 of the method ofthe present invention;

FIG. 12 is a schematic cross-sectional view illustrating a layerstructure of a top-gate bottom-contact TFT according to Example 3 of themethod of the present invention;

FIG. 13 is a schematic cross-sectional view illustrating a layerstructure of a bottom-gate top-contact TFT according to Example 4 of themethod of the present invention;

FIGS. 14A, 14B, 14C, 14D, 14E, 14F, 14G, 14H, 14I, 14J, and 14K areschematic cross-sectional views illustrating production steps of thetop-gate bottom-contact TFT according to Example 3 of the method of thepresent invention; and

FIGS. 15A, 15B, 15C, and 15D are schematic cross-sectional viewsillustrating the bottom-gate top-contact TFT according to Example 4 ofthe method of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, etching can be precisely controlledwhen producing a device in which different kinds (or a plurality) ofindium oxides coexist. Examples of the specific material to be etchedinclude those having a structure in which IZO and ITO, IGZO and ITO, IGOand ITO, IZO and IGZO, IZO and IGO, or IGZO and IGO coexist. By using asan etching liquid for etching such a structure, an etching liquid can beobtained which has selectivity for different substances.

Hereinafter, etching liquids used in the present invention will bedescribed in detail.

As acetic acid for use in the present invention, commercially availableacetic acid liquid (100% by weight of glacial acetic acid; density of1.05; hereinafter referred to as “stock solution”) may be used as such,or may be diluted with pure water in an amount, up to 4 times the volumeof the stock solution and then used. In order to maintain a high etchselectivity (about 2.8 or more and 4.3 or less) of IZO to IGZO, it ismore desirable to dilute the stock solution with pure water in an amount0.5 to 2 times the volume of the stock solution (concentration: 34% ormore and 68% or less). The etching process using the acetic acid isperformed by immersing an etching object in an aqueous acetic acidsolution at room temperature (about 20° C.).

Next, organic acids are described. Usable organic acids are not limitedto citric acid, and examples thereof include any generally known organicacids, such as malonic acid, malic acid, tartaric acid, oxalic acid,formic acid, glycolic acid, and maleic acid. A ligand existing in anorganic acid, for example, COO^(—) is combined with In to form a complexion under specific conditions and is dissolved. Hereinafter, descriptionwill be made by taking citric acid as an example. The citric acid is asolution prepared by dissolving commercially available citric acid(citric acid monohydrate; chemical formula: C₃H₄(OH)(COOH)₃.H₂O; whitesolid crystal) in pure water completely. The citric acid concentrationof an etching liquid containing citric acid of room temperature (about20° C.) is desirably 3.8% by weight or more and 45% by weight or less.In order to maintain a high etch selectivity (about 26 or more and 55 orless) of IGO to ITO, the citric acid concentration is more desirably3.8% by weight or more and 29% by weight or less. When the citric acidconcentration is less than 3.8% by weight, the etch rates of all oxidescontaining indium are lowered, so that the etch selectivity isremarkably reduced. When the citric acid concentration is more than 45%by weight, there are cases where citric acid crystals may beprecipitated in an aqueous etching solution containing citric acid.

The temperature of the etching liquid containing citric acid isdesirably within the range of 20° C. or more and 55° C. or less, andmore desirably within the range of 40° C. or more and 55° C. or less.When the aqueous citric acid solution is at about 50° C., the etchselectivities of the indium oxides including IZO, IGZO, and IGO to ITOdo not significantly change, but the etch rate increases byapproximately one order of magnitude from that at room temperature. Inthis case, in order to make the concentration constant, a unit forcooling, condensing, and collecting water that has evaporated from theetching liquid needs to be provided.

It is desirable that the citric acid concentration of the etching liquidcontaining citric acid of 50° C. be within the range of 3.8% by weightor more and 45% by weight or less. In order to maintain a high etchselectivity (from about 7.3 to 9.6) of IZO with respect to IGZO, it ismore desirable that the citric acid concentration be within the range of16% by weight or more and 45% by weight or less. In view of the aboveexperimental results, it is most desirable that the citric acidconcentration of the etching liquid containing citric acid at atemperature within the range of room temperature to 55° C. be within therange of 16% by weight or more and 29% by weight or less. Within theabove concentration range, the etch selectivities of the indium oxidesincluding IZO, IGZO, and IGO with respect to ITO are the highest andstable, without depending on the etching liquid temperatures.

As the hydrochloric acid, commercially available concentratedhydrochloric acid (containing 37.2% by weight of hydrogen chloride andhaving a density of 1.18, which will hereinafter be referred to as“stock solution”) may be used as such or may be diluted with pure waterin an amount 60 or less times the volume of the stock solution and used.In order to highly and stably maintain the etch selectivities of theindium oxides including IZO, IGZO, and IGO with respect to ITO, it isdesirable that the hydrochloric acid stock solution used for the etchingliquid be diluted with pure water in an amount 4 to 60 times the volumeof the stock solution (concentration: 0.7% by weight or more and 8.5% byweight or less). It is more desirable that the stock solution be dilutedwith pure water in an amount 4 to 40 times the volume of the stocksolution (concentration: 1.0% by weight or more and 8.5% by weight orless). In order to make easy the control of etching time, the mostdesirable hydrochloric acid concentration is obtained by diluting thestock solution with pure water in an amount 10 to 30 times the volume ofthe stock solution (concentration: 1.4% by weight or more and 4.0% byweight or less). The etching process using hydrochloric acid isperformed through immersion in an aqueous hydrochloric acid solution ofroom temperature (about 20° C.).

As the perchloric acid, commercially available concentrated perchloricacid solution (concentration: 70% by weight; chemical formula: HClO₄;density: 1.673; hereinafter referred to as “stock solution”) may be usedas such or may be diluted with pure water in an amount 20 or less timesthe volume of the stock solution. In the etching liquid containingperchloric acid, a desirable perchloric acid concentration is obtainedthrough dilution with pure water in an amount 1 to 20 times the volumeof the stock solution (concentration: 5% by weight or more and 44% byweight or less). The most desirable perchloric acid concentration isobtained through dilution with pure water in an amount 2 to 10 times thevolume of the stock solution (concentration: 10% by weight or more and32% by weight or less). By adding pure water of such quantity to thestock solution, the etch rate notably increases as compared with that ofthe stock solution. The etching process using perchloric acid isperformed through immersion in an aqueous perchloric acid solution ofroom temperature (about 20° C.).

By the use of the above-mentioned etching liquids, the etch rates of theindium oxides can be set so as to be IZO>IGZO>IGO>ITO.

According to the present invention, the production yields ofsemiconductor devices when using the oxide films containing indium, suchas IZO, IGZO, and IGO as an active layer and an electrode can beremarkably improved. In particular, such high production yield isimportant in the case of large area substrates.

Further, in a desirable embodiment, film deposition may be performed inan atmosphere containing oxygen gas without intentionally addingimpurity ions for increasing the electric resistance of the oxide thinfilm containing indium. By adjusting the oxygen concentration and thefilm deposition temperature, the electric resistance of the oxidecontaining indium can be adjusted.

Incidentally, the oxide films containing indium are produced by a thinfilm deposition process selected from sputtering process, vapordeposition process, CVD process, epitaxial growth process, photo induceddeposition process, pulse laser vapor deposition process, and ionplating process. Of those, the sputtering process is most suitable fromthe viewpoint of mass productivity.

Because the conductivities of the indium oxide films including IZO,IGZO, and IGO strongly depend on temperature, it is desirable not toincrease the temperature as far as possible in a wet etching process. Asthe temperature increases, the conductivities of the oxide filmscontaining In, Ga, and Zn increase. Therefore, the above-mentioned wetetching process temperature is desirably set to 60° C. or less.

In addition, in a TFT using the indium oxide thin film including IZO,IGZO, and IGO, the resistance of the film can be adjusted by filmdeposition conditions, so that the oxide film can be utilized not onlyas an active layer but also as pixel wiring for liquid crystals.Incidentally, as the material of the oxide film containing indium, atleast one of impurities such as Sn, Al, Sb, Cd, Ge, P, As, N, and Mg canbe added to compounds including IZO, IGZO, and IGO.

Further, when using a plastic resin substrate, it is desirable to use anetching liquid containing a weak acid (organic acids such as acetic acidor citric acid), or hydrochloric acid or perchloric acid with a lowerconcentration. This is because degradation may increase with increase ofthe etching temperature. In particular, when a substrate is made of aresin such as polyimide, polyethylene terephthalate, orpolyethylenenaphthalate, the acid resistance thereof is insufficient.Therefore, it is desirable to use hydrochloric acid or perchloric acidthat has been diluted with pure water. A desirable hydrochloric-acidconcentration at room temperature is obtained by diluting the stocksolution with pure water in an amount of 4 to 60 times the volume ofhydrochloric acid stock solution. Since immersion in a short period oftime is also desirable, a more desirable hydrochloric acid concentrationat room temperature is obtained by diluting the stock solution with purewater in an amount 10 to 30 times the volume of hydrochloric acid stocksolution.

Incidentally, when using the plastic resin substrate, a desirableperchloric acid concentration at room temperature is obtained bydiluting the stock solution with pure water in an amount 1 to 20 timesthe volume of perchloric-acid stock solution. A more desirableperchloric acid concentration at room temperature is obtained bydilution with pure water in an amount 2 to 10 times the volume ofperchloric-acid stock solution. By immersing the above-mentioned resinsubstrate in the dilute hydrochloric acid or dilute perchloric acidhaving a concentration within the above-mentioned desirableconcentration range for a short period of time (e.g., for 15 minutes),the above-mentioned resin substrate becomes insusceptible to notabledegradation or decomposition such as dissolution and swelling.

It is known that the above-mentioned acid etching liquid, i.e., solutionof any one of acetic acid, organic acid, hydrochloric acid, andperchloric acid, cannot etch a gate insulation film such as a siliconnitride film, which is generally used commonly. In place of the siliconnitride film, silicon oxide or dielectric materials such as siliconoxynitride, HfO₂, HfAlO, HfSiON, or Y₂O₃ can be applied to a TFT deviceas the gate insulation film without being etched by the above-mentionedacid etching liquid.

EXAMPLES

Hereinafter, the present invention will be described according to thefollowing examples with reference to the attached drawings, but is notlimited to the following examples. In each of the drawings, the shape,dimension, and arrangement of each constitutional element are roughlyshown in such a manner that the present invention can be understood.

In the present invention, when it is described that a layer (or a layerstructure obtained by patterning a layer) is formed on another layer ora substrate, it is to be understood that the layer may be formeddirectly on the another layer or the substrate, or a third layer may beprovided therebetween.

Example 1 Etch Rate Measurement

In Example 1, a specific example of an etching method of an indium oxidefilm including IZO, IGZO, IGO, and ITO of the present invention will bedescribed.

First, experimental samples are produced according to a proceduredescribed below. A Si substrate (525 mm in thickness) having a 100 nmthick Si thermal oxide film provided thereon is used as a base. Threeoxide films of an IGZO film, an IZO film, and an IGO film are formed onthe Si substrate with the Si thermally oxide film by reactive sputteringunder the conditions shown in Table 1 below.

Incidentally, indium oxide thin films of IGZO, IZO, and IGO eachcontaining microcrystals are deposited on the substrate by theabove-mentioned spattering film deposition method. Considering that theincidence X-ray diffraction (angle of incidence: 0.5°) of the IGZO, IZO,and IGO thin films shows no clear diffraction peak, it can be said thatthe produced IGZO, IZO, and IGO thin films are amorphous.

As a glass substrate with ITO, a commercially available glass substratewith a polycrystalline ITO film of 29 nm in thickness is used.

TABLE 1 Material IGZO IZO IGO O₂ partial 3.40% 0.97% 0.97% pressure Filmforming 0.5 0.4 0.4 pressure (Pa) Film thickness ~563 nm ~393 nm ~325 nm

Next, a line-and-space resist pattern with a line width of 100 micronand a space width of 100 micron is formed on the IGZO, IZO, IGO, and ITOthin films by a known photolithography method. The resist used is apositive resist AZ1500 (trade name; 20 cp) manufactured by Clariant. ASi substrate with the resist is hard baked at a temperature of about120° C. using a baking furnace to thereby obtain samples for etchingexperiments. The hard bake process is carried out for improving theadhesion of the resist during wet etching to be performed later and theresistance against an etching liquid.

Next, deionized water (DI water) was added to each of the commerciallyavailable reagents shown in Table 2 below to adjust the concentration toa desired value. The samples for etching experiments were immersed inthe concentration-adjusted etching liquids to thereby etch the IGZO,IZO, IGO, and ITO thin films exposed from the resist pattern.

TABLE 2 Specific Chemical Stock Solution Gravity of Reagent FormulaConcentration Liquid Acetic acid CH₃COOH 100% (glacial 1.05 acetic acid)Citric acid C₃H₄(OH) White crystal Solid 1-hydrate (COOH)₃•H₂O (Citricacid monohydrate) Hydrochloric HCl 35 to 37% 1.18 acid Perchloric HClO₄70% 1.673 acid

Incidentally, it is desirable to heat the etching liquid in a waterbath. In order to maintain the concentration, evaporation from anetching liquid is suppressed by use of a lid, and evaporated water isfrequently replenished. After etching is completed, the resist of eachsample is stripped with acetone. Then, the patterns of the indium oxidefilms are measured and observed. The step difference produced by theetching is measured with a surface profiler (trade name: Alpha-Step) andellipsometer manufactured by KLA-Tencor Corporation, and the etch rateis accurately computed.

FIG. 1 is a graphical representation illustrating the acetic acidconcentration dependence of the etch rates of IZO, IGZO, IGO, and ITOwhen an aqueous acetic acid water solution is used at room temperature(ordinate: etch rate (nm/min); abscissa: volume ratio of diluting purewater to acetic acid stock solution). FIG. 2 is a graphicalrepresentation illustrating the acetic acid concentration dependence ofthe etch selectivities of IZO, IGZO, and IGO with respect to ITOobtained by converting the data of FIG. 1 (ordinate: etch selectivity toITO; abscissa: volume ratio of diluting pure water to acetic acid stocksolution). As shown in FIGS. 1 and 2, when the acetic acid concentrationis within the range from that of the stock solution to that of asolution prepared by dilution with pure water in an amount 4 or lesstimes the volume of the stock solution (concentration: 20% by weight ormore and 100% by weight or less), the etch rates of the indium oxidescan be stably set so as to be IZO>IGZO>IGO>ITO. When the pure watercontent of the etching liquid further increases, the etch selectivity islost and the etch rate is not practical. In order to maintain a highetch selectivity (about 2.8 or more and 4.3 or less) of IZO to IGZO, itis more desirable to dilute with pure water in an amount of 0.5 to 2times the volume of the stock solution (concentration: 34% or more and68% or less). Further, because the vapor pressure of acetic acid ishigh, it is desirable to, for example, inhibit its evaporation from theetching liquid by use of a lid or the like.

FIG. 3 is a graphical representation illustrating the citric acidconcentration dependence of the etch rates of IZO, IGZO, IGO, and ITOwhen an aqueous citric acid solution is used as an organic acid at roomtemperature (ordinate: etch rate (nm/min); abscissa: % by weightconcentration of citric acid). FIG. 4 is a graphical representationillustrating the citric acid concentration dependence of the etchselectivities of IZO, IGZO, and IGO to ITO obtained by converting thedata of FIG. 3 (ordinate: etch selectivity to ITO, abscissa: % by weightconcentration of citric acid). Usable organic acids are not limited tocitric acid, and usable are any generally known organic acids, such asmalonic acid, malic acid, tartaric acid, and oxalic acid. Citric acid isused in this example. As shown in FIGS. 3 and 4, the citric acidconcentration of a room-temperature (about 20° C.) etching liquidcontaining citric acid is desirably 3.8% by weight or more and 45% byweight or less. In order to maintain a high etch selectivity (about 26or more and 55 or less) of IGO to ITO, the citric acid concentration isdesirably 3.8% by weight or more and 29% by weight or less. When thecitric acid concentration is less than 3.8% by weight, the etch rates ofall the indium oxides are lowered and the selectivities remarkablydecrease. When the citric acid concentration exceeds 45% by weight,citric acid crystals precipitate in the aqueous etching solutioncontaining citric acid.

FIG. 5 is a graphical representation illustrating the citric acidconcentration dependence at 50° C. (±5°C.) of the etch rates of IZO,IGZO, IGO, and ITO (ordinate: etch rate (nm/min); abscissa: % by weightconcentration of citric acid). FIG. 6 is a graphical representationillustrating the citric acid concentration dependence at 50° C. of theetch selectivities of IZO, IGZO, and IGO to ITO obtained by convertingthe data of FIG. 5 (ordinate: etch selectivity to ITO; abscissa: % byweight concentration of citric acid).

Comparing FIGS. 3, 4, 5, and 6, the etch rates of the indium oxidesincluding IZO, IGZO, IGO, and ITO increase at near 50° C. byapproximately one order of magnitude as compared to that at roomtemperature, and the etch selectivities with respect to ITO alsoincrease slightly. Therefore, in order to make the etch selectivityhigher, the temperatures of the etching liquid containing citric acid isdesirably 20° C. or more and 55° C. or less, and more desirably 40° C.or more and 55° C. or less. As is seen from in FIGS. 5 and 6, the citricacid concentration of the etching liquid containing citric acid of 50°C. is desirably 3.8% by weight or more and 45% by weight or less. Inorder to maintain a high etch selectivity (from about 7.3 to 9.6) of IZOto IGZO, the citric acid concentration of the etching liquid containingcitric acid is desirably 16% or more and 45% by weight or less. In viewof the above experimental results, it is most desirable that the citricacid concentration of the etching liquid containing citric acid be notless than 16% by weight and not more than 29% by weight within thetemperature range of from room temperature to 55° C. In the aboveconcentration range, the etch selectivities of the indium oxideincluding IZO, IGZO, and IGO to ITO are most stable without depending onthe temperature of the etching liquid.

FIG. 7 is a graphical representation illustrating the hydrochloric acidconcentration dependence of the etch rates of IZO, IGZO, IGO, and ITOwhen an aqueous hydrochloric acid solution is used at room temperature(ordinate: etch rate (nm/min); abscissa: volume ratio of diluting purewater to hydrochloric acid stock solution). As is seen from FIG. 7, asthe hydrochloric acid concentration increases, the etch rate becomeshigher. However, it is difficult to control etch selectivity and etchingend point. FIG. 8 is a graphical representation illustrating thehydrochloric acid concentration dependence of the etch selectivities ofIZO, IGZO, and IGO to ITO obtained by converting the data of FIG. 7(ordinate: etch selectivity to ITO; abscissa: volume ratio of dilutingpure water to hydrochloric acid stock solution). As is seen from FIGS. 7and 8, in order to stably maintain high etch selectivities to ITO of theindium oxides including IZO, IGZO, and IGO, the hydrochloric-acidconcentration of the etching liquid containing hydrochloric acid isdesirably obtained by diluting with pure water in an amount 4 to 60times the volume of the stock solution (concentration: 0.7% by weight ormore and 8.5% by weight or less). For ease of the control of etchingtime, it is more desirable to obtain the hydrochloric acid concentrationof the etching liquid containing hydrochloric acid by diluting with purewater in an amount 4 to 40 times the volume of the stock solution(concentration: 1.0% by weight or more and 8.5% by weight or less). Inorder to facilitate the control of the etching time, the most desirablehydrochloric acid concentration is obtained by diluting with pure waterin an amount 10 to 30 times the volume of the stock solution(concentration: 1.4% by weight or more and 4% by weight less).

FIG. 9 is a graphical representation illustrating the perchloric acidconcentration dependence of the etch rates of IZO, IGZO, IGO, and ITOwhen an aqueous perchloric acid solution is used at room temperature(ordinate: etch rate (nm/min); abscissa: % by weight concentration ofperchloric acid). FIG. 10 is a graphical representation illustrating theperchloric acid concentration dependence of the etch selectivities ofIZO, IGZO, and IGO to ITO obtained by converting the data of FIG. 9(ordinate: etch selectivity to ITO; abscissa: % by weight concentrationof perchloric acid). As is seen from FIGS. 9 and 10, in order to obtainhigh etch selectivities to ITO, a desirable perchloric acidconcentration is obtained by dilution with pure water in an amount 1 to20 times the volume of the stock solution (concentration: 5% by weightor more and 44% by weight or less). The etch rate markedly increases byadding pure water of the above-mentioned amount to the stock solution,as compared with that of the stock solution. However, the etch ratetends to be reduced due to an excessive amount of pure water. Therefore,the most desirable perchloric acid concentration is obtained by dilutionwith pure water in an amount 2 to 10 times the volume of the stocksolution (concentration: 10% by weight or more and 32% by weight orless).

The wet etching experimental results of Example 1 show that the etchrates of the oxides almost stably have the relationship ofIZO>IGZO>IGO>ITO. Therefore, it can be seen that in the respectiveetching liquids, the selectivities of IZO to IGZO, IGZO to IGO, or IGOto ITO are lower than the relative selectivities of other oxides. Morespecifically, the critical value (minimum etch selectivity) of the etchselective abilities of the above-mentioned etching liquids resides inany one of the three etch selectivities of IZO to IGZO, IGZO to IGO, andIGO to ITO. Table 3 shows the ranges of the selectivities of IZO toIGZO, IGZO to IGO, and IGO to ITO for the respective etching liquids.

TABLE 3 Citric Acid Citric Hydro- Per- Etch Acetic (Room Acid chloricchloric Selectivity Acid Temperature) (50° C.) Acid Acid IZO/IGZO1.6-4.3 5.4-14.8 0.58-9.6  1.0-2.3 3.2-12.9 IGZO/IGO 3.9-5.5 3.8-10.4 5.3-21.7 4.3-5.9 3.1-6.0  IGO/ITO  78-140 9.0-55   18-83 166-396 50-570

As shown in Table 3, the selectivities of IGO to ITO are each at leastone digit value, and all the aforementioned etching liquids can beapplied thereto. In the case of IGZO to IGO, the use of the roomtemperature citric acid (e.g., citric acid concentration of about 45% byweight) enables to give etch selectivity of one or more digits, and ismore desirable. The use of the citric acid of 50° C. (e.g., citric acidconcentration of about 9% by weight) is the most desirable. In the caseof IZO to IGZO, the use of the room temperature citric acid (e.g.,citric acid concentration of about 29% by weight) enables to give etchselectivity of approximately one digit, and thus is more desirable.Further, the use of the perchloric acid enables to give higher etchselectivity (e.g., at near perchloric acid concentration of about 70% byweight), and thus is the most desirable.

As described above, the selectivity of the etching liquid variesaccording to the combination of the material to be etched and a matrixmaterial. When etching a structure having an amorphous oxide layercontaining In and at least one of Ga or Zn, it is necessary to selectetching conditions, such as etching liquids, concentrations, andtemperatures in accordance with the combination of the materials.According to the findings of the present inventor, the followingconditions are desirable, for example.

(1) When performing selective etching of a layer structure including anITO layer and an oxide layer including IZO, IGZO, and IGO formed on theITO layer, desirable conditions are within the following ranges.

-   (Room temperature) Acetic acid concentration: 20 to 68% by weight    (water to acetic acid stock solution: 0.5 to 4 times);-   (Room temperature) Citric acid concentration: 3.8 to 45% by weight,    more desirably 3.8 to 29% by weight;-   (50° C.) Citric acid concentration: 16 to 45% by weight;-   (Room temperature) Hydrochloric acid concentration: 0.7 to 8.5% by    weight (water to hydrochloric acid stock solution: 4 to 60 times),    more desirably 1.0 to 8.5% by weight (water to hydrochloric acid    stock solution: 4 to 40 times), most desirably 1.4 to 4% by weight    (water to hydrochloric acid stock solution: 10 to 30 times);-   (Room temperature) Perchloric acid concentration: 5 to 44% by weight    (water to perchloric acid stock solution: 1 to 20 times).

(2) When performing selective etching of a layer structure including anIGO layer and an oxide layer including IZO and IGZO formed on the IGOlayer, desirable conditions are within the following ranges.

-   (Room temperature) Acetic acid concentration: 20 to 68% by weight    (water to acetic acid stock solution: 0.5 to 4 times);-   (Room temperature) Citric acid concentration: 3.8 to 45% by weight;-   (50° C.) Citric-acid concentration: 16 to 45% by weight;-   (Room temperature) Hydrochloric acid concentration: 0.7 to 8.5% by    weight (water to hydrochloric-acid stock solution: 4 to 60 times),    more desirably 2.0 to 8.5% by weight (water to hydrochloric acid    stock solution: 4 to 20 times);-   (Room temperature) Perchloric acid concentration: 5 to 44% by weight    (water to perchloric-acid stock solution: 1 to 20 times), more    desirably 5 to 21% by weight (water to perchloric-acid stock    solution: 4 to 20 times).

(3) When performing selective etching of a layer structure including anIGZO layer and an oxide layer including IZO formed on the IGZO layer,desirable conditions are within the following ranges.

-   (Room temperature) Acetic acid concentration: 34 to 68% by weight    (water to acetic acid stock solution: 0.5 to 2 times);-   (Room temperature) Citric acid concentration: 3.8 to 45% by weight;-   (50° C.) Citric-acid concentration: 16 to 45% by weight;-   (Room temperature) Hydrochloric acid concentration: Unsuitable;-   (Room temperature) Perchloric acid concentration: 21 to 70% by    weight (water to perchloric acid stock solution: 0 to 4 times).

Example 2 Stack Structure

FIGS. 11A and 11B are cross-sectional views illustrating the most commonetching form according to an example of the present invention. A layer 1is a material layer with a lower etch rate, a layer 2 is a materiallayer with a higher etch rate, and a layer 3 (resist layer) is used asan etching mask. In usual, when the selectivity of the layers 1 and 2 ofFIGS. 11A and 11B is low, the depth of excessively etched area Δd of thelayer 1 of FIG. 11B and an undercut amount Δw of the layer 2 are closeto each other, resulting in variation of thin film transistor devicesdue to etching. It can be seen that with the use of the etching liquidsof Example 1, the etch rates of oxides containing indium have therelationship of IZO>IGZO>IGO> and ITO. According to the presentinvention, when the layer 1 of FIGS. 11A and 11B is an ITO layer and thelayer 2 is one material selected from the group consisting of IZO, IGZO,and IGO, there can be obtained the highly selective etching effect suchthat the etch selectivity of the layer 2 to the layer 1 is of about twodigits or more. In this case, however, the etch selectivity obtainedusing citric acid is approximately of one digit. When the layer 1 ofFIGS. 11A and 11B is an IGO layer and the layer 2 is any one of the twomaterials of IZO and IGZO, there can also be obtained the highlyselective etching effect such that the etch selectivity of the layer 2to the layer 1 is of about one digit. When, in FIGS. 11A and 11B, thelayer 1 is an IGZO layer and the layer 2 is IZO, the selective etchingof IZO to IGZO can be performed. By performing the etching as describedin the above-mentioned examples, only the layer 2 can be etched and theetching can be precisely stopped at the surface of the layer 1.Therefore, the problems of the conventional etching methods, i.e., thevariation in the electrode resistances, can be solved, and thecharacteristics of TFT devices can be made uniform.

Example 3 Top-Gate Bottom-Contact TFT

FIG. 12 is a schematic cross-sectional view illustrating a structure ofa top-gate bottom-contact thin film transistor according to one exampleof the present invention. Reference numerals 4, 5, 6, 7, 8, and 9 denotea substrate (e.g., glass substrate), drain electrode, source electrode,active layer (channel layer), gate insulation film, and gate electrode,respectively. Reference character L denotes a channel length. FIGS. 14Ato 14K are schematic cross-sectional views illustrating production stepsof a top-gate bottom-contact thin film transistor as shown in FIG. 12using the etching liquid of Example 1 of the present invention. As shownin FIG. 14A, a glass plate having a thickness of 500 mm (manufactured byCorning; trade name: Corning 1737 glass; glass transition temperature:640° C.) is used as the substrate 4. A polycrystalline ITO film with athickness of 250 nm is formed on the surface of the substrate 4 byreactive sputtering, and the drain electrode 5 and the source electrode6 are patterned by dry etching. Next, as shown in FIG. 14B, an IGZOoxide film layer having a thickness of 100 nm is formed thereon as theactive layer (channel layer) 7 by reactive sputtering. The IGZO oxidefilm includes In—Ga—Zn—O, and the composition in a crystalline state isrepresented by InGaO₃(ZnO)_(m) (m is a natural number of less than 6).The transparent IGZO oxide film is a transparent amorphous semiconductoroxide film containing microcrystals therein and having an electroncarrier concentration of less than 10¹⁸/cm³.

Next, FIG. 14C illustrates a step of exposing contact pads for wiring ofthe drain electrode 5 and the source electrode 6 to the surface of theactive layer (channel layer) 7 and patterning the resist layer 3 forforming a channel. Next, as shown in FIG. 14D, this resist pattern isused as an etching mask, and the above-mentioned active layer (channellayer) 7 is etched with the etching liquid of Example 1, whereby thecontact pads of the drain electrode 5 and source electrode 6 areexposed. At this time, the present invention enables to performselective etching of the active layer 7 with respect to the drainelectrode 5 and source electrode 6. For example, as is seen from theexperimental results of Example 1, when dilute hydrochloric acid(concentration: about 4.0% by weight) diluted at room temperature withpure water in an amount 10 times the volume of hydrochloric acid stocksolution is used, the etch selectivity of IGZO to ITO is about 2000.More specifically, because the etch rate of IGZO is high and the etchrate of ITO is low, the etching can be performed in such a manner thatIGZO is greatly etched and ITO is hardly etched. More specifically, asshown in FIG. 11B, a two-layer structure of the ITO layer 1 and the IGZOlayer 2 is etched with the above-mentioned liquid by using the resist 3as an etching mask, whereby only the IGZO layer 2 is etched and theetching can be precisely stopped at the surface of the ITO layer 1. Asshown in FIG. 14D, when the selective etching of the active layer 7 withrespect to the drain electrode 5 and the source electrode 6 can beperformed, the etching depth d of the active layer can be preciselycontrolled. More specifically, the above described conventional etchingmethod of indium oxide film have problems of, for example, rise inelectrical resistance or damage to the electrode and wiring due to thedepth Δd of excessive etching in the drain electrode and sourceelectrode. The present invention can solve the problem of the variationin the device performances.

FIG. 14E illustrates a step of stripping the resist layer 3 with acetoneafter the selective etching of the active layer 7. FIG. 14F illustratesa step of forming a silicon nitride film (Si₃N₄) having a thickness of100 nm as the gate insulation film layer 8 by reactive sputtering on theuppermost surface of the upper side of the substrate 4. In place of thesilicon nitride film, silicon oxide, or dielectric materials such assilicon oxynitride, HfO₂, HfAlO, HfSiON, and Y₂O₃ can be used as thegate insulation film layer 8. As shown in FIG. 14G, the pattern of thegate insulation film layer 8 (silicon nitride film) is formed byphotolithography and reactive ion etching (RIE). In the RIE, the siliconnitride film is dry etched using fluorocarbon gas (e.g., CF₄). It isknown that indium oxides cannot be etched with ions or radicalsgenerated from the fluorocarbon gas by plasma. FIG. 14G illustrates anexample in which the active layer 7 is entirely covered with the gateinsulation film layer 8. However, in this process, the gate insulationfilm layer 8 does not necessarily have to cover the entire active layer7, and insulation may be made between the gate electrode layer 9 and theactive layer 7 in subsequent steps. More specifically, the gateelectrode layer 9 and the active layer 7 can be brought into contactwith each other. When the active layer 7 is exposed, the etchselectivity of the electrode layer 9 with respect to the active layer 7,the drain electrode layer 5, and the source electrode layer 6 may berequired in a subsequent step illustrated in FIG. 14J.

Next, as shown in FIG. 14H, an IZO film having a thickness of 100 nm isformed as the gate electrode layer 9 by reactive sputtering on the topsurface of the upper side of the substrate 4. As shown in FIG. 14I, apattern is formed on the gate electrode layer 9 using the resist layer 3as an etching mask by photolithography. As shown in FIG. 14J, the gateelectrode layer 9 is etched using an etching liquid containing citricacid to form the gate electrode 9. An etching liquid in which the weightratio of citric acid crystal powders to pure water is 40:100(concentration: 28.6% by weight) is used at room temperature (about 20°C.). The etch selectivity of IZO to IGZO is about 14.8, and the etchselectivity of IZO to ITO is about 1400. That is, also in the case wherethe active layer 7 is exposed from the gate insulation layer 8 using theetching liquid containing citric acid, selective etching of the gateelectrode layer 9 (IZO) to the active layer (IGZO) and simultaneously tothe drain electrode layer 5 (ITO) and the source electrode layer 6 (ITO)can be performed. When using a hydrochloric acid solution (moleconcentration: 6 M) diluted with pure water in an amount equal to thevolume of hydrochloric acid stock solution, the etch selectivity of IZOto the IGZO is as low as about 1.06, and selective etching of the gateelectrode layer 9 (IZO) to the active layer (IGZO) cannot be performed.

Finally, as shown in FIG. 14K, the resist layer 3 is stripped withacetone. A top-gate bottom-contact thin film transistor is obtainedwhose electrodes and active layers all have oxide films containingindium (ITO drain electrode layer 5, ITO source electrode layer 6, IGZOactive layer 7, and IZO gate electrode 9).

Example 4 Bottom-Gate Top-Contact TFT

FIG. 13 is a schematic cross-sectional view illustrating a structure ofa bottom-gate top-contact thin film transistor according to anotherembodiment of the present invention. Reference numerals 4, 5, 6, 7, 8,and 9 denote a substrate (e.g., glass substrate), drain electrode,source electrode, active layer (channel layer), gate insulation film,and gate electrode, respectively. Reference character L denotes achannel length. FIGS. 15A, 15B, 15C, and 15D are schematiccross-sectional views illustrating production steps of a bottom-gatetop-contact thin film transistor as shown in FIG. 13 using the etchingliquid of Example 1 of the present invention. An IGO layer (gateelectrode layer 9) having a film thickness of 100 nm is formed byreactive sputtering on the surface of a glass plate with a thickness of500 mm (manufactured by Corning; trade name: Corning 1737 glass; glasstransition temperature: 640° C.) as the substrate 4. A resist ispatterned on the surface of the gate electrode layer 9 (IGO) as anetching mask by photolithography. Next, the gate electrode layer 9 (IGO)is patterned with dilute hydrochloric acid etching liquid having aconcentration of 4.0% by weight and the resist is stripped. Next, asilicon nitride film (Si₃N₄) having a film thickness of 100 nm formed byreactive sputtering is patterned as the gate insulation film 8 on thesurface of the gate electrode layer 9 by photolithography and dryetching. FIG. 15A is a schematic cross-sectional view illustrating thestate in which the resist has been stripped after the dry etching. Aswith the case of the top-gate bottom-contact TFT, in place of thesilicon nitride film, silicon oxide or dielectric materials such assilicon oxynitride, HfO₂, HfAlO, HfSiON, and Y₂O₃ can be used as thegate insulation film layer 8. For attaining the subsequent wiring andgate voltage application, the gate electrode 9 is not completely coveredwith the gate insulation film 8 but a part of the gate electrode 9 needsto be exposed as a contact pad. The contact pad of the gate electrode 9is provided in a direction perpendicular to the drawing plane of FIG.15A, but is not shown in FIG. 15A.

Next, an IGZO layer having a film thickness of 100 nm is formed thereonas the active layer (channel layer) 7 by reactive sputtering. The IGZOoxide film includes In—Ga—Zn—O, and the composition thereof in acrystalline state is represented by InGaO₃(ZnO)m (m is a natural numberof less than 6). The IGZO oxide film is a transparent amorphoussemiconductor oxide containing microcrystals therein and having anelectron carrier concentration of less than 10¹⁸/cm³. Next, the resistlayer 3 is patterned on the surface of the active layer 7 (IGZO) as anetching mask by photolithography. When using an etching liquid in whichthe weight ratio of citric acid crystal powder to pure water is 10:100(concentration: 9.09% by weight) at about 50° C., the etch selectivityof IGZO to IGO is about 21.8, so that selective etching can beperformed. That is, as shown in FIG. 15B, even if a part of the contactpad of the gate electrode 9 (IGO) is actually exposed using the etchingliquid containing citric acid, selective etching of the active layer 7(IGZO) can be performed. In this case, if an etching liquid having noselectivity is used, the gate electrode 9 is etched. As a result, theetching liquid is permeated into the interface between the gateelectrode 9 and the gate insulation film 8, which makes the channelwidth of a TFT device smaller than the actually designed value. Thisposes problems that the ON-current of the TFT device is reduced, it isdifficult to control variation in the device performance, and massproduction cannot be achieved.

Next, the resist is stripped and an IZO film having a thickness of 250nm is formed as a drain electrode and a source electrode by reactivesputtering on the uppermost surface of the upper side of the substrate4.

Then, the resist layer 3 is patterned on the surface of the IZO film asan etching mask by photolithography. Next, as shown in FIG. 15C, usingan etching liquid containing citric acid, the IZO film is etched to formthe drain electrode 5 and source electrode 6. In actual, the contact padof the gate electrode layer 9 (IGO) is exposed as described above, to bebrought into close contact with the IZO film. Thus, the selectiveetching of IZO to IGO is required. When using an etching liquid in whichthe weight ratio of citric acid crystal powder to pure water is 40:100(concentration: 28.6% by weight) at room temperature (about 20° C.), theetch selectivity of IZO to IGZO is about 14.8, and the etch selectivityof IZO to IGO is about 56. That is, using the etching liquid containingcitric acid, selective etching of the drain electrode layer 5 (IZO) andsource electrode layer 6 (IZO) to the active layer 7 (IGZO) andsimultaneously to the gate electrode layer 9 (IGO) can be performed.However, if an etching liquid having no selectivity is used in thisstage, the gate electrode 9 is etched. As a result, the etching liquidis permeated into the interface between the gate electrode 9 and gateinsulation film 8 which makes the channel width of a TFT device smallerthan the actually designed value. Moreover, the active layer 7 is alsoetched, and the etching liquid is permeated into the interface betweenthe drain electrode 5 and source electrode 6, and active layer 7. As aresult, the channel length of a TFT device becomes larger than theactually designed value. In some cases, there are posed the seriousproblems that the active layer continues to be etched, so that due tothe shortage of channel depth, the channel resistance is increased andthe ON-current of a TFT device is markedly reduced, whereby the devicecannot be operated. If the drain electrode layer 5 and source electrodelayer 6 are formed by the lift-off process in this stage, there is posedthe problem that when the photoresist is stripped, an edge of a patternof a deposited film is curved up, and thereafter an electric wirecrossing over the pattern edge is liable to be broken.

Finally, as shown in FIG. 15D, the resist layer 3 is stripped withacetone. Thus, a bottom-gate top-contact thin film transistor isobtained in which all the electrode and active layers are formed ofoxide films containing indium (IZO drain electrode layer 5, IZO sourceelectrode layer 6, IGZO active layer, 7, and IGO gate electrode 9).

Industrial Applicability

The present invention can provide a highly selective etching method ofoxide semiconductor films containing In, Ga, and Zn and formed on asubstrate, and also can form semiconductor devices having stable anduniform electrical characteristics on a substrate.

For example, a TFT in which a transparent oxide film is used as anactive layer can be applied onto a soft plastic film, and also can beapplied to the fields of pixel drivers of flexible displays, IC cardsfor authentication, product ID tags, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. An etching method of an amorphous oxide in a structure comprising anITO layer and an amorphous oxide layer, the method comprising etchingthe amorphous oxide layer, which comprises In and Ga, In and Zn, or In,Ga and Zn, using an etchant comprising any one of acetic acid, citricacid, hydrochloric acid, and perchloric acid at an etch rate larger thanan etch rate of the ITO film.
 2. An etching method of an amorphous oxidein a structure comprising an oxide layer comprising In and Ga and anamorphous oxide layer, the method comprising etching the amorphous oxidelayer, which comprises In and Zn or In, Ga, and Zn, using an etchantcomprising any one of acetic acid, citric acid, hydrochloric acid, andperchloric acid at an etch rate larger than an etch rate of the oxidelayer comprising In and Ga.
 3. An etching method of an amorphous oxidein a structure comprising an oxide layer comprising In, Ga, and Zn andan amorphous oxide layer, the method comprising etching the amorphousoxide layer, which comprises In and Zn, using an etchant comprising anyone of acetic acid, citric acid, hydrochloric acid, and perchloric acidat an etch rate larger than an etch rate of the oxide layer comprisingIn, Ga and Zn.
 4. The etching method of an amorphous oxide according toany one of claims 1-3, wherein a concentration of the acetic acid is 20to 68% by weight, a concentration of the citric acid is 3.8 to 45% byweight, a concentration of the hydrochloric acid is 0.7 to 8.5% byweight, and a concentration of the perchloric acid is 5 to 44% byweight.